Typical automotive power steering systems include a fluid reservoir, an engine driven pump, a servo-valve connected to the steering wheel, and a steering assist fluid motor. When the engine is on, the pump continuously circulates working fluid in a closed loop starting and ending at the reservoir. Because the fluid level in the reservoir varies normally from minimum when the fluid is cold, i.e. at ambient temperature, to a maximum when the fluid is hot, the reservoir fill cap is usually vented to the atmosphere. To avoid fluid leakage through the vent and to minimize cavitation at the pump inlet, closed or sealed power steering systems have been proposed. For example, U.S. Pat. N. 4203287, issued to J. L. Bennett and assigned to the assignee of this invention, describes a power steering system wherein the reservoir is a sealed, expansible chamber connected to the return line from the servo-valve to the pump. The fluid circuit, including the reservoir, is completely filled with fluid at atmospheric pressure when the fluid is cold. As the temperature of the fluid increases during operation, the volume of the expansible chamber expands to accommodate thermal expansion of the fluid. A power steering system according to this invention is a closed system wherein the fluid is pressurized for minimizing pump cavitation and wherein the fluid level in the reservoir is below a fill port for easy fluid replacement.